Magnetic control of electric polarization in (Cu,Ni)B$_2$O$_4$*

We report the generation and control of electric polarization using an external magnetic field in a noncentrosymmetric system nickel doped copper metaborate (Cu,Ni)B$_2$O$_4$ and discuss the origin of this effect. In this material, weak ferromagnetic moment can be rotated by applying an electric field. While this implies spin-driven ferroelectricity, the previous study via examining the structure parameters and dielectric constant showed no clear evidence for this effect, which is successfully observed in this study. Applying a magnetic field along the $[110]$ or $[1-10]$ axis induces electric polarization along the $[001]$ axis. The polarization is reversed by switching the magnetic field direction between the $[110]$ and $[1-10]$ axes. The result can be well explained in the framework of spin-dependent metal-ligand hybridization.